Neutron-Proton-Converter Acceleration Mechanism at Subphotospheres of Relativistic Outflows

We study a type of particle acceleration that operates via neutron-proton conversion in inelastic nuclear collisions. This mechanism can be expected for relativistic shocks at subphotospheres if relativistic outflows contain neutrons. Using a test-particle approximation, we numerically calculate the energy spectrum and the efficiency of accelerated particles, and show that a good energy fraction of the nucleons can be accelerated. This mechanism may especially be relevant if the shock is radiation-mediated, and it would enhance the detectability of GeV-TeV neutrinos.